As an ultra high speed communication device suitable for an application such as a cellular phone and optical communication, the development of a heterojunction bipolar transistor (HBT) using a heterojunction material such as SiGe (Silicon-Germanium) and GaAs (Gallium-Arsenic) has proceeded.
Japanese Patent Application Laid-open publication No. 2001-85446 (Patent Document 1) discloses an HBT in which a laminating film including a silicon film and an SiGe film is used as a base layer and a metal silicide is adopted to a base ohmic layer, thereby reducing the resistance of an external base electrode.
The base layer mentioned above is formed of a laminating film which comprises a seed layer made of a silicon film, an SiGe layer added with no dopant, a p+-type SiGe layer, and a silicon layer added with no dopant. The external base electrode comprises a first external base electrode obtained by doping a high concentration of boron into a laminating film made of the same materials as those of the base layer and a second external base electrode which is selectively grown on the first external base electrode. The second external base electrode includes at least one of a silicon film, an SiGe layer, and a Ge layer, and is doped with boron. A base ohmic layer made of a metal silicide film is formed on the second external base electrode in order to reduce the resistance of the external base electrode.
Japanese Patent Application Laid-open publication No. 2000-124222 (Patent Document 2) discloses an HBT in which a high frequency characteristic is improved by reducing the resistance and the parasitic capacitance of a base region. This HBT comprises a base region made of a boron doped SiGe epitaxial layer formed on the surface of a collector region and an external base electrode made of a boron doped polysilicon film. The base region is electrically connected to the external base electrode through a second boron doped polysilicon film and a third polysilicon film. The third polysilicon film contains boron in high concentration which diffuses from the second boron doped polysilicon film and the external base electrode. In this structure, since the distance between the external base electrode and the collector region becomes large, the parasitic capacitance of the base region can be reduced. In addition, since dopant concentration in the third polysilicon film becomes high, the resistance of the base region can be reduced.
Japanese Patent Application Laid-open publication No. 2003-45884 (Patent Document 3) discloses an HBT in which a high concentration of boron is ion-implanted into the connection portion between a base layer and an external base electrode in order to suppress a parasitic resistance between the base layer and the external base electrode, thereby achieving high speed and lower noise.